In 2010, the Department of Homeland Security seized 96 tons of marijuana at the US-Mexico border. At airport security checkpoints, the TSA confiscates 40 firearms (often loaded) everyweek. And for as long as anyone can remember, the 700-ft, icy monstrosity that is the Wall, has kept out White Walkers—even if the occasional wildling manages to scale it quite easily. So what are the properties of an effective barrier? What are the mechanisms that allow passage to certain things, while restricting everything else? In the case of the brain, it’s MFSD2A.